This invention relates to electric arc processes, and is particularly, but not necessarily exclusively, concerned with such processes as are used underwater.
Various electric arc processes are used underwater, by divers, mainly for welding and cutting. Most required the application of an open circuit voltage somewhat higher than the operating arc voltage. Thus a typical welding arc voltage of, say, 20 or 30, volts may use a supply with an open circuit voltage of 50 volts or more.
A code of practice exists which specifies that the maximum exposed voltage that can be used by divers in this situation is 30 volts. It then allows that this may not be practical and therefore so-called safe methods of working have to be used In practice divers who use this equipment are accustomed to experiencing electric shocks, but as they are, by experience unpleasant rather than life threatening, they are tolerated as a part of the process.
Plasma arc cutting, with typical arc voltages in the range 100-200 volts and open circuit voltages of 200-400 volts have not generally been used by divers underwater as those levels of voltage are more likely to prove life threatening. Because of the conductive nature of water there are many potential paths for currents to flow through the water and a divers body, and it is difficult to distinguish between a current which is flowing safely, and one which is likely to be dangerous.
In general terms most arc processes require two electrodes, one of which is often the workpiece, and the other usually contained within an item called a torch or electrode holder (hereinafter called xe2x80x9ctorchxe2x80x9d). In order to produce an arc underwater a gas must be supplied to it, or it must be produced xe2x80x9cin situxe2x80x9dxe2x80x94e.g. by creating steam, or by the effect of heat on the electrode material. The resulting ionised gas in the arc is highly conductive compared to water.
Usually the workpiece is connected to the other side of the arc power supply and so forms the other electrode (transferred arc processes). Often in situations above water this is also connected to earth for safety reasons. In the situation underwater the workpiece can frequently be considered xe2x80x9cthe earthxe2x80x9d since it is likely to influence the electrical potentials experienced in the water near to it, and to which a diver might be exposed.
There could be situations where different floating objects and the sea bed could be at different potentials and thus the xe2x80x9cearthxe2x80x9d referred to is a relative one, not xe2x80x9cabsolutexe2x80x9d.
There is also the situation of the non-transferred arc process, in which the torch contains both electrodes. In this situation it is likely that the workpiece will still be considered as earth in that it is likely to influence the potential in the water nearby. It is often the case, in plasma cutting processes, for the arc to be initiated in non-transferred mode (pilot arc) but for the cutting to take place in transferred mode.
With normal arc welding processes used underwater the most dangerous situation is probably when the supply is applied to the torch, but an arc is not struck. In this situation the applied voltage is generally at its highest, and the torch can be positioned away from the workpiece (earth). As is illustrated schematically in FIG. 1 of the accompanying drawings this opens up the possibility of exposing the diver directly to the open circuit voltage or to a part of it should the diver accidentally come into contact with the workpiece or be positioned between the torch and the workpiece.
The situation with non-transferred process is similar in that the non-transferred arc in the torch can impart a potential to the surrounding water, with respect to the xe2x80x9cearthxe2x80x9d. As is shown schematically in FIG. 2 of the accompanying drawings, this can put at risk a diver in the water between the torch and the workpiece.
The object of the present invention is to provide for the safe operation of plasma arc processes underwater, or any other process where voltage must be applied in a conductive medium and hence could give rise to the electric shocking of an operative.
According to the present invention, underwater electric arc equipment in which an electric arc is struck as between a primary electrode and a workpiece, through the intervening conductive medium of water comprises an earthed safety electrode positioned in relation to the primary electrode of the equipment to create the electrically closest earth to which current could flow through the conductive material from the primary electrode without the striking of an arc between the primary electrode and the earthed safety electrode.
In a preferred form of construction, the earthed safety electrode may be an annular ring or cylinder, concentrically positioned in relation to the torch, and whereby an arc struck between a torch electrode and a workpiece, passes through the annular earthed electrode.
Desirably, a torch body is provided to house the primary electrode and is either insulated or insulated and fitted with an earthed enclosure. In many cases the earth torch body may form sufficient of a safety electrode even if it does not obviously enclose the arc. However, the closer the safety electrode is to the point at which potentials are exposed to the water, the less likely it is that potentials will be experienced in the water outside the region of the torch. Thus the form of the safety electrode is not material to this invention, but rather its position in relation to the primary electrode and the arc (or other similar process), and to the diver and the workpiece.